LOCALIZATION OF CHROMOSOME BREAKPOINTS INDUCED BY DNASE-I IN CHINESE-HAMSTER OVARY (CHO) CELLS

DNase I was electroporated into S-phase CHO cells and induced chromoso me breakpoints were localized in G-banded metaphases. More than 75% of breakpoints mapped to Giemsa light bands, 18% to Giemsa-dark bands an d about 7% to band junctions. Chromosome breakpoint clusters produced by DNase I colocalized with chromosome breakpoints induced by the rest riction endonucleases AluI and BnmHI in the G1- and S-phases of the ce ll cycle in CHO cells, Digestion of metaphase spreads with AluI, BamHI and DNase I produced G-bands, indicating that G-light bands are more sensitive to endonuclease action. The possible role of nuclease-sensit ive sites in active chromatin as selective targets for the induction o f chromosome breakpoints by these endonucleases is discussed.